This invention relates to a method of making superconducting ribbon, by disposing a yttrium or rare earth metal-alkaline earth metal-copper-silver alloy ribbon between metal sheets, rolling the composite, and then oxidizing the metal mixture to form oxides.
Perovskite related ceramic oxides, comprising alkaline earth metal-copper oxide, such as orthorhombic, yttrium-barium-copper oxide materials, usually characterized as YBa.sub.2 Cu.sub.3 O.sub.7-x or "1:2:3 ceramic oxides", are well-known "high temperature" superconductor materials. This 1:2:3 ceramic oxide material has been found to provide electrical superconductivity, i.e., essentially no electrical resistance, at temperatures less than or in the region of 93.degree. K.
The 1:2:3 ceramic oxides and other rare earth metal-alkaline earth metal-copper oxide based ceramics can operate in the superconducting mode near the 77.degree. K. boiling point of relatively inexpensive and plentiful liquid nitrogen, and could find increased use in composite windings for high current magnets, energy storage coils, long distance power transmission, and the like. However, 1:2:3 ceramic oxide ribbons and other superconducting ceramic oxide ribbons generally made from sintered component oxide particles, are hard and brittle, and are not easily fabricated into fine wire or windings.
This brittleness was recognized by Jin et al., in Applied Physics Letters, "High T.sub.c Superconductors-Composite Wire Fabrication", Vol. 51, No. 3, Jul. 20, 1987, pp. 203-204. As a solution to this problem, Jin et al. placed a metal cladding around a heat treated 1:2:3 ceramic oxide powder superconducting core. The metal cladding, Ag, or Cu with a Ni/Au oxygen diffusion barrier, allowed ease of drawing into fine wire form, from 0.6 cm to 0.02 cm diameter, and also provided an alternate electrical conduction path in case the ceramic oxide lost its superconducting properties, i.e., became normal or resistive. Ag was found particularly advantageous as a cladding, since it could act as a dual cladding and oxygen donor. The drawn wires were then annealed at 900.degree. C. and 600.degree. C. in oxygen. Multifilamentary ribbon composites were also formed. Jin et al. also recognized the problem of oxygen loss from the metal clad 1:2:3 ceramic oxide, suggesting addition of an oxygen donor to the core, use of a perforated or porous cladding, and the like.
Attempts to solve problems with ceramic oxide superconductor brittleness have been reported by Wessel and Stipp in The Wall Street Journal, June 11, 1987; by Hilsdorf in Metal Working News, p. 23, Aug. 17, 1987; and by Robinson in Science "A New Route to Oxide Superconductors", Vol. 236, No. 4808, p. 236, June 19, 1987; all involving an apparent M.I.T. process of making a melt of europium, barium, copper, and possibly gold as reported by Hilsdorf, to form a metal alloy, and spinning the melt on a rotating wheel, to produce a very thin, solid ribbon made up solely of the alloy, followed by a controlled oxidation heat treatment in an oxygen environment to form a superconducting oxide ribbon.
A similar method of alloying Eu, Ba and Cu, solidification processing; and oxidizing to form the cuprate, is also reported by Haldar et al., in Applied Physics Letters "EuBa.sub.2 Cu.sub.3 O.sub.x Produced By Oxidation Of A Rapidly Solidified Precursor Alloy: An Alternative Preparation Method For High-T.sub.c Ceramic Superconductors", Vol. 51, No. 7, pp. 538-539 (1987). There, a Eu.sub..167 Ba.sub..333 Cu.sub..500 alloy was prepared by arc melting. The alloy was then quenched into 25 micrometer thick foil. The foil was heated at 900.degree. C. in pure O.sub.2 followed by cooling to 25.degree. C. over 4 hours, to form the desired cuprate. Haldar et al. noted the apparent difficulty at that time of preparing rare earth (other than Eu or Yb) -Ba.sub.2 Cu.sub.3 O.sub.x superconductors by a metal-alloy route.
Forming a Y-Ba based 1:2:3 ceramic oxide by an alloy route was apparently solved by Vander Sande and Yurek, as reported by Dragani in Chem. & Engrg. News, Vol. 66, No. 49, p. 5 (Dec. 5, 1988), by forming a precursor alloy of yttrium, barium, and copper with a noble metal such as silver or gold, and heating in O.sub.2 to form a 1:2:3 ceramic oxide, plus noble metal which is not oxidized. Yurek and Vander Sande suggest, in U.K. Patent Application GB 2,202,528A, published on Sept. 28, 1988, that a superconducting precursor mix of La, Ba and Cu; La, Sr and Cu; Y, Ba and Cu; or Eu, Ba and Cu, can have a metal selected from Au, Pt, Pd, Ag, or an excess of one of the metal constituents of the mix, added to and melted with the precursor composition. An alloy can be formed as a coating or as wire, ribbon, sheet, rod, or ring using chill casting or the like. The alloy can then be oxidized to convert the precursor elements to a superconducting oxide without oxidizing the metal additive. The metal additive (Au, Pt, Pd, Ag, etc.) is continuous throughout the ceramic and provides improved mechanical properties, such as ductility and strength, acting as a "skeleton" in the ceramic.
Others have formed 1:2:3 ceramic oxides by first melt spinning a YCu.sub.3 alloy into a thin ribbon, then dip coating it with a molten mixture of BaO.sub.2, BaCO.sub.3 and Ba(OH).sub.2 at 550.degree. C., followed by stepped heating in air to 925.degree. C., as reported by Pinkerton et al. in Applied Physics Letters, "Superconducting Yttrium-Barium-Copper-Oxide Ribbons Fabricated From A Metal Alloy Precursor", Vol. 53, No. 5, pp. 438-440 (Aug. 1, 1988). The product, however, contained a small grained core of Y.sub.2 BaCuO.sub.5 and CuO.sub.x within the YBa.sub.2 Cu.sub.3 O.sub.7-x superconducting structure.
What is needed is a method of optimizing the making of superconductor ribbons, where the ribbons have superior properties of deformability and oxygen permeability deep into the core. It is a main object of this invention to provide such a method.